Fundamentals for the Aspiring Assassin
by Spontaneite
Summary: One day, reality splits about four metres to Nagisa's left. [Time travel, fix-it] [In-progress, crossposted on ao3/tumblr; 4 chapters available as of 10/2017]


Fundamentals for the Aspiring Assassin

Chapter 1

Nagisa was the first to notice when reality split apart, most likely because it happened directly beside him.

It was just a glimpse. Something appearing in his periphery that hadn't been there a moment ago. Distracted, he turned his head towards it, knife lowering from the ready stance, and saw it. Just a glimpse, where it looked like light was bending and fracturing, like the air itself was paper, like something was pushing through-

A thunderclap resounded directly beside him, shockingly loud, and hot winds from nowhere blasted him off of his feet along with most of his assassination volleyball team. Nagisa heard alarmed screams and yells erupting around him, and he just barely made use of his lessons to take the fall properly, wincing at the impact of his leg and arm on the ground. Still, it wasn't a bad fall, and he was among the first springing back to their feet after the unexpected blow. A small storm of dust had been whipped up by whatever-it-was, particularly where it had originated. Nagisa squinted in the direction he'd been thrown from, and could just about make out the sight of something…metallic?

"What the hell was that?" Maehara screeched, on behalf of probably everyone. Nagisa looked towards his voice, waving the dust from in front of his face. The redhead was on the other side of the net, which incidentally had been blown over. It looked like he'd been caught under the net, and had perhaps narrowly avoided a pole to the face. Terasaka had been caught by the net as well, but it seemed like everyone else on that side had been clear. On his side, though…

"Is everyone okay?" Karasuma-sensei demanded, rushing over from the sidelines. "What happened? …Did someone detonate a bomb?" It was a marker of their weird classroom that this seemed a perfectly sensible question to ask in such a circumstance.

Unexpected sort-of explosions weren't so unusual, here. And, well…Despite the grenade incident being quite far in the past, everyone still looked at Nagisa. He wilted slightly under the stares. "Ah…Not me?"

From the floor, Okajima squinted at him. "You were closest to where it went off." He was only now picking himself up, which was either an indication that he needed more falling practice or that he had a terrible reaction time. Maybe both.

"I saw something just before," Nagisa mumbled, and looked back to the thing he'd seen. The dust was still swirling, catching unpleasantly in his throat as he breathed, but it had calmed enough to view the shape of the mystery object. "And there's something there." He resorted to pointing, because apparently looking at him and Karasuma-sensei was so interesting that no one else had thought to look towards the source of the impromptu wind-blast yet.

Obligingly, twenty-seven sets of eyes looked to the side. It would have been twenty-eight, but Karma was skipping – he always seemed to miss the interesting things. The dust obligingly settled into a restless stirring by the ground as they stared, revealing what certainly seemed to be an extremely large suitcase. Or maybe a safe with handles? It was a large, heavy-looking thing, with a hard shell and expensive-looking lock that covered a fair portion of one of its sides. A thing which had, apparently, appeared from nothing.

"…It's a suitcase." Sugino observed, baffled, as the other half of the team pulled the net off of their fellows and rushed over to investigate. " A _suitcase._ That wasn't there a minute ago, right?" he looked around, almost pleadingly. "Right?"

"It wasn't." Isogai agreed, staring at it. For a moment, no one spoke or moved, more than slightly flummoxed by the spectacle. Then, shrugging, Maehara stepped forwards and strode up to the suitcase.

"Stop." Karasuma-sensei's voice cut sharply across the field, even though he didn't speak especially loudly. Maehara froze in place along with all the rest of them – their teacher had a pretty commanding presence to him, after all. He pushed through the crowd, waving Maehara out of the way, and cautiously regarded the case from three metres away. "A suitcase just appeared from _thin air._ That is not normal." He pointed out dryly. "It would be irresponsible for me to allow any of you to come into contact with it. It could be dangerous." Off to the side, Kurahashi swooned a little.

Nagisa blinked, rubbing a residual ache out of his arm as he watched.

"Back up. All of you." The man added, crossing his arms. He stared out impassively at them until, reluctantly, the closer students backed away. His stare remained until they were all a good ten metres away – excessive, by some standards, but Nagisa could see the logic. If it were a bomb of some sort, for example, ten metres might not be nearly far enough.

Karasuma-sensei sighed at them, then turned warily to the case. He stared at it for several seconds in silence. Nagisa wondered what he was looking for. Triggers? Signs of a countdown? All he could see from this distance was the lock on the side, which seemed to have some sort of intricate mechanism on it, even a small screen. It looked more like something you'd expect to find on a high-security door than a suitcase.

For that matter…the suitcase looked _very_ well-armoured. Nagisa would flat-out call it a large safe if not for the obvious wheels and handles. He shared a bemused glance with Kayano, wondering what Karasuma-sensei would do with it. Would he approach it himself, or was he too wary? Would he call his bosses before approaching, or investigate himself?

"Nagisa-kun." Karasuma-sensei said, out of nowhere. Nagisa looked up, startled. "What do you know about this?"

And, suddenly, everyone was looking at him again. "…Sensei?" He ventured, taken aback.

"The case has your name on it." Their assassination instructor informed him, plainly. He pointed to the locking mechanism, presumably at a set of characters too distant to make out.

… _What?_ Nagisa stiffened, his first inclination to deny it, but...Karasuma wasn't really the sort to joke about anything. "I-it does?" Why on earth would a suitcase appearing from thin air have _his name_ on it? The idea was _absurd._ His next breaths came too quickly for his liking, and he clamped down on the panic with rigid control.

"'Shiota Nagisa'." Karasuma-sensei read out, slow and measured. "You can come and stand beside me to read it, if you don't believe me."

Feeling the weight of his class' eyes on him, Nagisa swallowed, forcing his muscles to loosen. After a careful breath, he walked out from beside his classmates and approached his teacher, peering forwards. From this distance…the characters stood out in relief, plain and dark on the metallic lock. His eyes widened.

"Is it really your name, Nagisa-kun?" Nakamura called across the meagre distance. He glanced back, and found everyone stewing in varying flavours of curiosity and impatience.

"Ah…it is." He confirmed, probably a bit too quietly for the furthest people to hear him. He nodded, hesitantly, if only to confirm it to himself. His name was on a spontaneously appearing suitcase. What on earth was someone meant to do in such a situation?

"I will ask again, Nagisa-kun. Do you know anything about this?"

Nagisa looked back at Karasuma-sensei. He was, admittedly, quite fond of the man. He was a good teacher, professional, and had respect for his students. It was nice to have the honest regard of an adult like him. He had no desire to disappoint the man. The only thing he could really do was be honest, even if the truth was difficult to believe. "I don't know anything, sensei. I think it appeared next to me, but…" he trailed off, shaking his head. "Sorry."

Karasuma regarded him calmly for several moments, then nodded. "Alright. Well then, what to do about this…"

Nagisa relaxed, very slightly, at the indication that he'd been believed. "Karasuma-sensei, do you think we should get Korosensei to look at it? If it's dangerous, he's least likely to be hurt." And, naturally, if it was dangerous and he _was_ hurt, all the better for their assassination.

"Quite true." Karasuma nodded, pensive. "Logically, however, this is related to that octopus or assassination of him – something so unusual happening here is unlikely to be unrelated. If it _is_ related to assassination, it could be a bad idea to let the target see it. It could be a weapon, and secret weapons are best kept hidden."

"Ah…" Nagisa dipped his head, frowning lightly. "Who would send us a weapon by teleportation without leaving something more to go on?"

"Who would send _you_ a weapon, Nagisa-kun." The man pointed out. "It has your name on it."

"Is there anything else written on it?" They both turned at the sound of Maehara's voice, calling across the distance mandated by their instructor. Both of them absorbed the question and turned to investigate.

Nagisa scrutinised the case. It was at least a metre long, laying on its front with a liberal coating of dust and dried grass. It was also thick enough that the lock was basically a large metal plate on its armoured side, where the large characters of his name were written. But on the screen, which was darker, something could conceivably be written in small letters. "Can I go closer to see, sensei?" he asked, after a moment.

Karasuma scrutinised the case and sighed. "It hasn't exploded yet, so I will go closer. You stay here and be ready to run." He said, and walked forwards to close the meagre distance, kneeling down to read. He paused. "Hm."

"Sensei?" Nagisa inquired, noticing that his classmates were inching closer in what they probably thought was a stealthy manner. He made a face at them, receiving several rude faces in return. Maehara even stuck out his tongue.

"It says 'present thumbprint'." Karasuma said, and after a second did precisely that, demonstrating none of the caution and restraint they might have expected from him.

"Karasuma-sensei!" Nagisa, plus at least four of his classmates, protested – far too late to be of any use.

"Hm!" Karasuma expressed, this time sounding quite surprised. He looked at Nagisa, and then frowned, eyes fixing on the troupe of disobedient students that were certainly quite a bit closer now. "I said to stand clear!" he snapped.

"You're letting Nagisa near it, sensei." Okajima pointed out, dropping the pretence of stealth and flat out striding over to the classmate in question.

" _Risking one student may be preferred to the whole class_." Ritsu's voice observed, at a slightly-too-low volume. Nagisa looked over and saw Kayano holding out the host phone. _"However, this item isn't showing any signs of imminent combustion, and hasn't moved since it arrived. It may prove dangerous, but most likely not until it is opened."_

Kayano pressed her finger to the volume button on the side of her phone as she asked "Opened?"

" _The display is a fingerprint scanner."_ Ritsu stated brightly. _"It must be connected to a locking mechanism."_

"Did it reject your fingerprint, sensei?" Isogai inquired, coming to stand beside Nagisa.

"…It did." Karasuma confirmed, and shifted out of the way to allow their view of the little screen. "Nagisa-kun, see for yourself."

Nagisa leaned forwards and squinted to read it. "'Incorrect print. Two attempts remaining.'" He said aloud, narrowing his eyes. "...sensei?"

"It has your name on it." The teacher stated, looking directly at him. "Will you try?"

He swallowed. "It might explode."

"It might." Karasuma acknowledged. "If yours doesn't work, I'll call my bosses and try to have it opened by professionals. It seems sensible to try your print first, however."

Nagisa looked at his classmates, uneasy. As expected, all of them were staring at him. Some seemed a bit too eager, considering the potential danger of this. He looked back. "I'll do it." He said, settling slowly into the calm resolve that assassination called for.

With the same serious, respectful nod as he had that time, Karasuma stood up and stepped back. "Good. Everyone back up again – all the way to the school steps. If it does explode, there's no sense in us all dying." There was a fair bit of grumbling at that, but a sharp glance and a sharper word cut through it. "Now!"

"Harsh, sensei." Terasaka grumbled, rolling a shoulder as he walked away with the others. "Oi, Nagisa, try not to blow up."

Nagisa sighed, appreciating the irony of being told that by someone who'd once given him a grenade to suicide-bomb their teacher with. "Aa."

Karasuma-sensei watched with a hawk-like eye until he deemed all of the students sufficiently distant. "Alright then." He said, and Nagisa took it as the go-ahead it was. He shuffled forwards, kneeling down by the case to stare at his name, and the little screen. _'Incorrect print. Two attempts remaining.'_ It read, unchanging. He exhaled, and reached out, pressing the thumb of his right hand firmly to the screen.

A moment passed. A light bleep sounded, and across the screen characters flashed – _'Print accepted – Shiota Nagisa'_ – and then the front panel….hissed? There was a hissing sound, and a brief click. He removed his thumb, hesitant, and the panel immediately lifted away, springing up slightly on a mechanised hinge. He flinched back, wary of the potential of dire contents – but the case didn't open. Only the front of the lock.

"Nagisa-kun?" Karasuma questioned, kneeling beside him.

"It took my print." He murmured, carefully lifting the panel further up to see what was beneath. There was…another screen, a larger one, as well as a number pad. The screen bleeped, and text appeared on it. "…'Broadcasting'?"

"What does it mean by that?" Karasuma-sensei muttered, as if expecting Nagisa to know the answer.

"Sensei!" A voice called from a good distance, all the way over by the school. Kayano?

Karasuma didn't look, eyes fixed on the display. " _Stay there!"_ He ordered.

"Sensei!" Kayano called again, which was quite unusual for her. Nagisa looked over, and saw her brandishing her phone. "Something happened to Ritsu!"

At this first sign of trouble, Karasuma didn't seem especially inclined to abandon the case. He had shown regard for Ritsu in the past, Nagisa had noticed, but certainly less than to the other students. That sentiment quickly shifted when all of their phones abruptly bleeped, buzzed, or otherwise indicated that they were restarting and installing upgrades.

"All of the phones…?" Kayano murmured, as Nagisa and Karasuma reached the crowd. "This can't be a coincidence, can it?"

"Probably not." Karasuma answered, checking a good five phones before he confirmed that they were all still loading. "The screen in the case said 'broadcasting' – most likely, it has broadcast something to your phones which has disrupted them."

"What about Ritsu? Is she alright?" Nakamura demanded, staring at the screen of her phone with consternation.

"She had numerous hard copies. Even if her data is corrupted on every one of your phones, she should be fine."

"Sensei?" Kayano presented the screen of her phone outwards, demonstrating the lit-up display. "Mine is starting up again." Quite rapidly, the ranks of her classmates closed around her.

"Is it broken? Will it work?" Isogai asked the white light emanating from the screen, fretful. "I can't afford another phone, it has to work!" He stared at his own device, which was still loading, a nervous tremor in the fingers gripping it.

"I think mine is finishing, too." Okuda piped up, waving her smartphone.

"It's finished booting." Kayano said, quickly tapping in her passcode. Nagisa leaned over to watch as her phone desktop loaded, and a second later – "Ritsu is loading?" The screen had gone pink in the way that indicated Ritsu's processes taking over the phone.

A moment later, as had become quite familiar, the pink-haired avatar of an artificial intelligence commandeered the screen, a lush and verdant background appearing behind her. There was a noteworthy difference, though. "Ritsu?" Nagisa asked, incredulous, and heard a ripple of shock pass over his class.

The avatar blinked the same unusual eyes that they were used to. The hair was the same style, and the same colour, but the face was indisputably older. Ritsu looked like a young adult, still fresh and bright, but certainly not someone you could mistake for a fifteen-year-old.

She smiled. _"Good morning, everyone!"_ She chirped from the phone speakers. A moment later, she loaded on Okuda's phone, blinking from another screen, and took up her speech with two sets of speakers on the next sentence. _"It's wonderful to see you all again!"_

"What happened, Ritsu?" Nagisa asked her as she activated on his own phone, and Sugino's, and Kimura's… "All of our phones suddenly crashed – was it something to do with that case?"

All of the Ritsu-phones paused, simulating a slow blink. It seemed most of the phones had booted her up by now. When she spoke, the voice came from over a dozen phones, and honestly the effect was more than slightly eerie. _"Yes, it was. The case broadcasted data intended for me._ " She paused, which for her was a careful simulation of hesitance. " _The data provided upgrades. I have updated myself on all of your phones, and that is why I look different."_

"Ritsu." Karasuma-sensei spoke clearly and calmly, his voice instantly cutting through the chatter as always. "Did the data include information about the case?"

" _It did."_ Nagisa noted with amusement that Ritsu seemed to be present on twenty-six phones now, despite the fact that none of them were technically meant to have them during P.E class. The only person without one was their teacher himself.

When Ritsu did not elaborate, Karasuma frowned. "Please tell us everything you know."

" _I am not permitted to tell you that._ " Ritsu answered, far too cheerfully. A resounding silence fell at her flat-out, downright happy, refusal. Maehara whistled lowly, the sound carrying rather well in the hush.

"Why are you not permitted?" Karasuma demanded, stepping close to directly address Nagisa's phone.

The avatar appeared to consider the question for a moment. Considering her processing speed, this was almost certainly another artifice. " _The entirety of the information I have is only available to certain individuals under certain circumstances._ " She oriented her eyes towards Karasuma-sensei, most likely using the front camera as a reference. _"There are some things I can tell you all. Firstly, I can induce this case to explode. Also, if anyone attempts to force it, it will detonate. The blast radius will be about forty metres."_

Muttered curses swept around the clearing. The closest was Okajima, who uttered an impressed "Damn!" and inched backwards.

"Will anything else cause the case to explode?" Their special forces teacher asked, shoulders more intimidating than Nagisa was used to, and a rather severe expression settling onto his features. "Is there any other risk associated with it?"

" _I am installed in the case as a security precaution. If unauthorised individuals attempt to access the contents, or force their way in, I will make it explode."_ Ritsu informed him, as though she were discussing ice cream flavours instead of what was essentially a bomb threat. _"There are items inside the case which can cause considerable harm if handled improperly, so improper handling is not advised."_

"Why is my name on it? Why did it have my fingerprint?" Nagisa cut in, making all of the closest Ritsu-phones orient their eyes and faces in his direction. It was quite unsettling. Karasuma looked up at him, but didn't say anything against the question. Clearly, he considered it pertinent.

Ritsu beamed at him. _"That's because the case is for you, Nagisa-san!"_

Nagisa stared. "Um…"

"Why Nagisa?" Karasuma cut in, swiftly.

Her voice was almost sing-song in cadence. _"I am not permitted to say."_

Their instructor scowled. "Are you permitted to tell Nagisa, if the case is for him?"

Ritsu ruminated on the screen, averting her gaze demurely. A bird landed on her hair and chirped. _"Nagisa-san is allowed access to certain parts of the case, provided he follows instructions."_ She said, hunching a little, as though feeling shy. _"But…I can't tell you everything yet, sorry!"_

Karasuma sighed. "Right. And does this have anything to do with assassination?"

Ritsu's young adult avatar widened her eyes, blinking endearingly. _"Ah…I can't say."_

"What I really mean by that is, should we avoid letting that octopus know about this case? Would it decrease our chances of a successful assassination?"

Bizarrely, the question seemed to alarm her. _"I can't say. But...my opinion is that keeping Korosensei out of things is likely to be useful, at this stage."_

"Right." Karasuma-sensei nodded decisively. "Okay, everyone," he pitched his voice to carry. "Since this might be related to assassination, no one is to mention it to your octopus of a teacher. If he asks about the damage to the field, you will say that an experimental assassination tool caused the damage, and that you sustained only very light injuries. You will refuse to give him more details if he asks, because giving too much information to your target is a bad idea. You should also attempt to keep him distracted during afternoon classes, where possible." He turned to stare pensively at Nagisa. "Nagisa-kun, you'll be working with me. It seems I will need your help to determine the facts about this case."

"What if Korosensei asks where Nagisa-kun is?" Kayano asked, sensibly. "Nagisa-kun doesn't usually skip class, so he probably won't ignore it."

"Hmm." Karasuma frowned at Ritsu's face on the screen. "You will tell him that I am giving Nagisa a special assassination tutorial related to the experimental tool, and that he isn't allowed to interfere." He was silent for several moments, and then continued. "No one is to ask Ritsu about this when he might be listening. Ritsu, can you simulate your previous appearance in class?"

The image on the screens, between one moment and the next, reverted quickly to the fifteen-year-old Ritsu avatar. _"Of course, sensei!"_

"Then it's settled. I will be cancelling class to deal with this issue. You should exercise until the end of the period, don't waste this time. Don't practice with weapons without supervision." Karasuma glanced sharply towards Nagisa, who had to admit to feeling exceptionally on-edge by this point. "Nagisa-kun."

"Yes, sensei." He nodded, and fell in line with the teacher as he strode back towards the case, Ritsu still occupying his phone screen. "Ritsu, is the case safe to move?"

" _Yes, but handle it gently. There are fragile things inside._ " She paused, and added _"Isogai has sent you a message saying that the others want you to tell them anything you find out later."_

"Don't just say it out loud, Ritsu!" Nakamura shouted in the distance.

" _Ehehe. Sorry?"_

Karasuma emitted a very beleaguered sigh. Nagisa could sort of sympathise.

* * *

"Is there anything else you can say now that it's only us, Ritsu?" Karasuma asked, once he'd set the case carefully on the floor in his office. Nagisa hovered awkwardly in the corner, not especially comfortable in what was clearly a teacher-only space.

" _You do not have special permissions, Karasuma-sensei."_ Ritsu chirped from the phone, still in Nagisa's hand.

"What about me?" He inquired of the device.

" _You have special permissions, Nagisa-kun, but they will only be activated if you follow a particular set of instructions."_

Karasuma motioned at him to be quiet, and demanded "Who issued the instructions?"

Ritsu smiled. _"Nagisa did._ "

There was a very protracted silence. "Eh?!" Nagisa spluttered, unable to keep the shock internal, because…what? " _Me?"_

" _The contents of this case, and of the data upgrade I received, were assembled by you, Nagisa-san."_ The AI proclaimed, cheerfully indifferent to the emotional crises she was presenting him with. _"I am a security measure, and also a backup. If you decide not to carry out your instructions, I will detonate the case and carry out my own orders."_

"W-what instructions?" Nagisa asked, unable to keep quiet. A glance to the side revealed an extremely sceptical glower on his instructor's face. He didn't believe her, then? Or, at least, he thought there was foul play involved.

" _My own instructions are classified. Yours will likely take a while to complete."_

"What are the instructions for Nagisa-kun?" His teacher cut in, leaning a little closer to the phone than Nagisa was comfortable with. He quickly put the phone on the desk to make it more mutually accessible.

Ritsu's expression brightened. " _On the panel that opened on the side, you should see another screen. Please place and hold your finger on it, Nagisa-san. Until it tells you to remove it."_

Nagisa glanced at Karasuma, who held up a hand. Obediently, he remained still, and watched as his teacher interrogated what was technically another student. "Do these instructions pose any risk to Nagisa-kun?"

Ritsu was silent for at least three seconds, which was not reassuring. _"Completing the instructions will be physically unpleasant, and require an adjustment period. Full side effects are unknown. However, it is not expected that serious or permanent injury will result, provided the instructions are followed carefully."_

He eyed her with alarm. "What if the instructions aren't followed carefully?"

Ritsu made a face at him. _"Do you really need to know? If you're careful, it's not important."_

"You can't expect him to do something unless he knows the risks involved." Karasuma pointed out. "Especially if he doesn't know what he will gain from it."

" _Hmph!"_ Phone-Ritsu crossed her arms, looking quite put-out. _"Nagisa thought it was worth the risk, or he wouldn't have given me the instructions!"_

"Just give an overview of the benefits and the risks, Ritsu. Please." The man sighed, rubbing at his temples. Nagisa nodded fervently.

Ritsu stared out balefully from the screen. _"In the case, there are items which will allow Nagisa-san to learn many new things very quickly."_ She said. _"When the process is complete, he will have decades worth of information, including useful data on how to assassinate super-beings like Korosensei. He will also know everything about the case, including why and how he sent it."_

Nagisa exchanged a wide-eyed glance with his teacher. "That…sounds very useful." He said, a little reluctantly. "It also sounds like cheating, though."

" _That's silly, Nagisa-san._ "

"I agree. Information is an assassin's greatest weapon." Karasuma concurred quietly, the initial surprise settling into a considering frown. "And, if Ritsu is to be believed, you were the one to arrange all of this, despite how ridiculous it sounds." His voice implied, very strongly, just how ridiculous he found it.

Nagisa was quiet for a moment while he thought. "Ritsu," he said. "Tell me the biggest risks, please."

She pouted. _"Mou…well, the items necessarily act on your brain. If they are damaged, or interfered with improperly, they might do the wrong things and cause damage._ "

His heart gave a sudden, distinct, heavy thump in his chest. Nagisa swallowed, thoughts of his mother's violent mood swings searing through the front of his mind. "That sounds _horrible._ "

" _We were careful, you know._ " Ritsu said, plaintively. _"We don't want you to get hurt, so we made it as safe as possible. Please just open the case? I'll show you the items._ "

Nagisa looked uneasily at his superior. Carefully, the man nodded, and Nagisa exhaled. "…Alright." He approached the case and knelt down, flipping open the outer panel to expose the screen inside. He regarded it warily. Instead of 'broadcasting', it now read 'present your thumb, Nagisa!'. Moreover, the text was pink. Ritsu didn't seem to be lying about her presence in the case, then.

With trepidation, he pressed his right thumb firmly to the screen and kept it there. In short order, the screen heated up, the characters ' _analysing!'_ flashing at the top.

" _The screen will feel hot until it finishes, Nagisa-san. Please keep your thumb where it is."_

Nagisa winced as the heat, slowly but surely, became painful on his skin. He took a deep breath, and remained still. By the time the screen cleared, he felt like he'd seared the finger on a stove, and it throbbed painfully.

" _Thank you, Nagisa-san!"_ Ritsu chirped, and her words flashed on the screen in front of him. _"I can open the first portion of the case, now!"_

He backed away hastily as the central portion of the case's lid unexpectedly lifted away. He wondered what the number pad was for, if it just needed some other sort of print to open. "It has separate sections?" Maybe the keypad was for the other sections?

" _Only this first section is available until you have completed your instructions."_ Ritsu proclaimed. _"Have a look inside!"_

Karasuma knelt beside him as he pushed the lid back and looked.

The middle compartment was relatively small. The interior was extremely well-padded, with the shock absorption and thick walls taking up most of the space. Nagisa carefully lifted away a padding layer, and found a befuddling array of items.

There were two sturdy metal bottle of maybe half-litre capacity, and two syringes, complete with uncomfortably thick needles. There were also two metal cylinders. The largest item, though, was something that for all the world looked like a helmet. A thick metal helmet with _lots_ of wires, and also some sort of neck plating.

"What is he meant to do with this?" Karasuma asked, once enough time had passed with Nagisa unable to find his voice.

" _The bottles contain a fluid to be injected by the headset. The cylinders contain pills – one for pain, and the other to prevent bad reactions from your immune system."_ Ritsu explained. _"This process will take anything from three days to two weeks. The first time you equip the headset, it will carry out a comprehensive scan of your brain and determine how long the process will take. You should do that now, Nagisa-san – it won't take long."_

Nagisa stared at the headset that would apparently be manipulating his brain with no small amount of apprehension. "Will I be able to take it off?"

" _The headset is very advanced. For the scan, you will need to press a button and wait a few minutes to make it safe to remove. During the actual process, you can't take it off at all."_

He looked up. "…Sensei?"

Karasuma looked him directly in the eyes. There was no condescension there; not a hint of derision or cruelty. Only quiet, serious respect. "It's your choice, Nagisa-kun. Information is very valuable, but we know too little about this situation to have any guarantees about the risks."

Nagisa remembered another moment like this, another thought. _'If he's the one handing me the knife, I know I can trust him', right? But…_ "Ritsu, if somehow it _was_ myself issuing these instructions, I would find it easier to trust in them." He said, softly. "But it doesn't make sense. How could I have done all of this and not remember it at all?"

Ritsu blinked at him from the smartphone screen. _"Eh? You didn't figure it out?"_ She asked, surprised. _"Obviously it's time-travel!"_

A pause.

" _What?!"_ Both of them shouted it; not quite at the same time, but almost.

Ritsu huffed. _"What did you think I meant, when I said you'd sent me? Some sort of complicated plot where you had your memory wiped?"_ She rolled her eyes at them, as though time travel were any less ridiculous than complex memory-wiping schemes. _"Sending objects back in time is quite easy, but we've found no way to make it work on living creatures._ _Thankfully, artificial intelligences like myself are easy to send back! Especially for me, since I already had a past self I could hack and overwrite!"_ She looked exceptionally pleased with herself at that.

"S-so, when you say _I_ arranged this and wrote the instructions, you mean it was _me in the future?_ " Nagisa asked, feeling more shocked than he ever had in his life. He'd been quite sure that nothing would ever compare to the surprise of learning Class 3-E had become an assassination classroom. Clearly, he'd been very, very wrong.

" _Of course. I was an excellent candidate for time travel, but I am restricted to technology. You decided we needed to try restoring you, Nagisa-san. That's why this headset is primed with the details of your future brain._ " Ritsu pointed towards the side of the screen closest to the case. " _The technology has never been used to change an immature brain before, so we're being very careful. There are other treatments in the case we can use to help you recover, if things don't go well."_

"So when you say _decades of knowledge_ , that's decades of _future knowledge._ " Karasuma exhaled slowly.

"Does that mean we killed Korosensei, if the memories are from more than a year in the future?" Nagisa asked, not certain he actually wanted to know the answer. On one hand, he swelled with anticipation at the possibility of such success, but on the other…he thought it might seem sort of hollow, finding out like this.

Ritsu frowned at him, an unusual expression for her face. _"I can't tell you, Nagisa-san. You'll have to find out for yourself._ "

Karasuma looked at him. "…I hate to say it, Nagisa-kun, but if this involves _decades_ of memory, it's possible the problem is larger than your class' assassination." He pointed out. "Again, it's completely your choice what to do."

Nagisa stared into the case, thoughts and emotions at war. A large and sensible part of him was adamant that one should not risk permanent brain damage on the word of an artificial intelligence claiming borderline-unbelievable things. Unfortunately, the rest of him was quite stuck on the words _time travel,_ and was slightly too shocked to listen. He took a deep breath, and reached for the headset. "I can do the first scan, at least."

" _Put it on, and press the button on the middle of the forehead."_ Ritsu instructed, perking up at his apparent acquiescence.

The headset was heavy and cold in his hands. The interior was padded, and studded with odd ring-shaped pieces of metal. A large flap came down from the back of it, looking like it would envelop his neck, though he couldn't see the fastenings. Carefully, he lifted the headset and put it on, feeling his hair ties prickle against his scalp. "Should I have my hair up?" He asked, a little uneasy.

Ritsu paused. _"Do the ties contain any metal?_ "

"A little."

" _It's best you take them out, then."_

Nagisa removed the headset, setting it gently in his lap, and reached up. His hair, as much as he detested its length, was relatively well-behaved; it took mere seconds of careful tugging to pull out the ties, allowing it to fall softly around his neck. He glanced to the side, finding a trace of curiosity in Karasuma's eyes, and looked away. He put the headset on again, and reached up to his forehead to find the button. Pushing past his clamouring fears, he pressed it, and waited.

Abruptly, the neck-piece stiffened and closed around the sides of his neck, connecting at the front in a series of quiet clicks. Nagisa repressed the flash of panic at the feeling of pressure on his throat, and _waited._ He very determinedly did _not_ raise his hands to tear at the thing tightening on his neck, no matter how he disliked any sort of constriction near his airways.

The padding inside the headset seemed to expand, pressing against his scalp. It grew warm. The neck-piece tightened in several places, and loosened in a few others. Then, abruptly, he felt a stabbing pain at the back of his neck, like a thick needle. "You didn't say anything about needles, Ritsu." he muttered, clenching his fists. He shuffled to sit cross-legged on the floor beside the case, Karasuma kneeling seiza to watch him carefully, setting the Ritsu-phone beside him. The tightness of the neck-brace made it hard to move his head.

" _The headset is injecting a compound that will allow for more detailed scanning. This will take about fifteen minutes to complete._ " She informed him.

"…Okay." He made a face.

"Is everything alright, Nagisa-kun?" Karasuma requested, voice and face a little grim. He seemed tense. Was he worried?

"There was a sharp pain at the back of my neck. I can still feel it, but it's not as bad now." He answered, carefully keeping his hands still in spite of the urge to rip the helmet off. "…It's only fifteen minutes."

"This time." His teacher reminded him, bleakly.

Nagisa frowned at his lap. "…Yeah." He looked at the screen of his phone, his head far heavier than he was used to it being. "Ritsu…you said the actual procedure will take days. Did…did _we_ plan for how to do that? It's not like I could let my mother see this thing."

"… _Oh._ " Ritsu looked a little worried. _"The plan was just to have you go missing for the duration. I can access the headset's scans and supervise you, to make sure you're doing alright."_ She hesitated, and added _"But...having someone to help would be very useful. You should really be unconscious for the active parts, and I don't like the idea of you being vulnerable then."_

Nagisa looked at his teacher expectantly, receiving a light grimace in turn. "I could supervise, but not for very long." He said. "I can't justify significant absences without informing my bosses, and somehow I doubt you will want that."

" _Your bosses are very curious, Karasuma-sensei!"_ Ritsu said, bright and cheerful. _"If they try to interfere with the case, I will have to blow them up!"_ She sounded utterly unconcerned about the possibility.

Karasuma did not seem surprised in the least. "That is why I feel it would be a poor idea for me to be absent."

" _Aa…you might just have to mysteriously go missing then, Nagisa-san. I'll do my best to take care of you._ " Ritsu concluded, regretfully.

"I haven't agreed to do it, yet." He pointed out, closing his eyes. "Can you not tell me why this is important? It seems a lot of effort for me to go to when you know everything already. You could just tell me what I need to do."

She blinked wordlessly for several seconds. He thought that such a span might be remarkable – what was she processing, for it to take her so long? _"A lot of our plans will need someone with a body who knows the things that you do, Nagisa-san."_ She said, finally. _"Knowing isn't enough. You need to remember."_

"And you can't say _why?_ " Nagisa asked, tiredly, opening his eyes again to look at his phone. He'd barely had a few days to get used to the idea that, maybe, he actually had some talent for assassination – and then something like this was dropped on him? "Or even why it was _me?_ "

Ritsu, again, was quiet for an unusually long time. " _We edited my code so that I would have to obey the last orders you gave."_ She said. _"If you undergo the process successfully, I will be able to tell you anything. Until then…I only have a little leeway in my instructions. I can't tell you why you did this, or when, or how."_ A pause. _"But I can tell you that we did the bulk of our planning and research off-planet. I can also tell you that Earth would have had better facilities, with better resources."_ She waited.

Karasuma eyed her from above. "…Don't tell me – Earth was destroyed?"

" _I can't tell you that."_ Ritsu asserted.

"Can you tell me why you didn't do the research on Earth?" Nagisa prodded, feeling the neck-cover tighten again. He twitched against the impulse to raise his hand to it.

She smiled. _"No."_

"What planets were available as potential research bases?"

A pause. _"Mars was the only planet available."_ She presented them with two thumbs-up, the sunlight brightening in her background. It was not especially congruent to his mood.

Nagisa swallowed, tension bearing down on him. "So, something bad happened to Earth, whether or not it was destroyed." He concluded, heaviness settling into his veins. If that was the case, and his future self _had_ organised all of this, then…did he really have a choice? "I…suppose I have to, then. I have to try." In a way, it was reassuring, knowing what he needed to do. On the other hand, anything with brain damage as a potential consequence _terrified_ him.

"Is this so much more frightening than being told to kill a super-being of a teacher?" Karasuma asked, glancing at his watch.

"Korosensei isn't going to hurt me." Nagisa said plainly. "This could." He watched as the man's head dipped in acknowledgement, and looked down at Ritsu again. "You must have prepared for the idea that I wouldn't believe you about the time travel. Isn't there anything else you can tell me? If you have information, do you have things like pictures, too?"

Ritsu bleeped. _"We didn't think about the possibility of pictures."_ She said, sounding surprised. _"I have many in my database, yes, and some might not count as off-limits information. What would you like to see?"_

"A picture of whatever happened to Earth." He said, immediately.

Ritsu paused. _"I'm sorry, that isn't allowed._ "

Nagisa pondered it. "A picture of myself, then. As recent to the time travel as possible."

" _Retrieving data._ " A moment later, her image was replaced with a picture of him taken from the front, cropped so that only his head and shoulders were visible. Nagisa leaned in, fascinated.

He looked noticeably older. His hair was short – _yes, wonderful_ – and a little paler than he was used to. There were noticeable wrinkles around his eyes, and the eyes themselves…

He was smiling. The same smile he'd given to Takaoka-sensei, approaching with steel in his hand. He could read the earnest bloodlust in his own features, clear as day.

"How old was I?" He asked, after he'd considered the picture for a while. He didn't think he was any older than his mid-forties, but then, he'd always looked young. Maybe that persisted.

" _I can't say."_

"Figures." He sighed, and looked up at Karasuma. "What about a picture of the research facility?"

She appeared to ponder the idea. _"I think that counts as classified information about the research, sorry."_ She said, apologetically. _"Five more minutes until scanning is complete._ "

"What about a picture of where I live?"

" _That's the research facility."_

Nagisa blinked. "I'm a _scientist?_ " Even if he weren't in the end class, it was not a profession he'd ever considered. He supposed he put more effort into his observations of Korosensei than anyone else did, but that hardly seemed a basis for a research career.

" _That,"_ Ritsu said. _"Is-"_

"Classified?" He interrupted, resigned to the expected answer. She beamed at him. "Of course...I suppose we should just wait for the scan." Karasuma nodded at him, wordlessly, and they dropped into silence. It would have been completely quiet if not for the noises outside – his classmates yelling in the distance, birdsong from the roof, an odd scratching at the walls…

Nagisa turned to stare suspiciously at the outside wall, below the window. He caught Karasuma's eye and gestured minutely in the direction of the quiet sounds. The agent held the eye contact, followed it, and then inched quietly towards the window. He looked down.

A brief pause.

" _Get back to the field! No eavesdropping!"_ The sudden volume of his shout made Nagisa wince, glad that the headset covered his ears. He heard two voices screech a little in shock, and craned his head as best he could to see Maehara and Okajima fleeing in terror.

"…If they heard anything, they won't keep it quiet." Nagisa pointed out, a little wearily.

Karasuma breathed very slowly and deliberately for several seconds, irritation twitching his brow. "Ritsu, please let the students know that I'll be briefing them later, and that information leaks aren't to be tolerated. Be subtle."

" _Yes, sensei!"_ She chirped, and then presumably followed his instructions. It was hard to tell, given that she remained precisely where she was on the phone's display.

The remaining minutes passed quickly. Nagisa had never been so grateful for a beep as the one that sounded from the headset, directly before the neck brace released him. His hand went up instantly, touching lightly at the skin of his throat. "Can I take it off?"

" _Yes._ "

He didn't wait for further instructions, pulling the thing off. His hair had become sweaty and unpleasant, sticking to his neck. There was also a small smudge of blood on his finger when he pulled it away from the injection site.

"So, how long will the procedure take?" Karasuma asked while Nagisa set to work tying his hair back up. There was a dull ache where the needle had been.

Ritsu hummed. _"Including rest cycles, it will take six days."_ She said. _"After that, you will be able to remove the headset. However, it is likely you will need a further recovery period of at least two days."_

Nagisa grimaced. "Finals are only weeks away…"

" _If this works, Nagisa-san, you will easily get the best mark in a number of things. If nothing else, you are fluent in English._ "

Karasuma raised an eyebrow. "Useful." He commented.

"Well, that's a definite benefit." Nagisa murmured. "How can we do this, sensei?" He put the headset carefully back into its case, and stood up with his phone.

"…It might be difficult." He said. "Depending on how often you need supervision."

" _If you make sure to keep your phone charged, Nagisa-kun, I can try to remind you of when to take your medication. And I can call people if you need outside help."_ Her avatar looked towards Karasuma. _"Sensei, could you supervise Nagisa-san during the active modification cycles?"_

"How long are the active cycles?" He asked.

" _Five hours. With twelve hour recovery before the next active session."_

Karasuma held his hand to his mouth, brows furrowed in thought. "If he stays at my house, I can supervise evening and night cycles. Anything else, though…"

" _Aa."_ Ritsu nodded. _"It will have to do. Maybe the rest of the class can help?"_

Their teacher grimaced. "And then I'll have you all knowing where I live. Lovely." Taken aback at the sardonic almost-humour from such an unlikely source, Nagisa let out a surprised laugh. "At any rate, if we're going with the idea that Nagisa will vanish for a week, what will the story be when he returns?"

"…If it's anything traumatic, my mother might not react well." Nagisa admitted. "Even without that…I don't think she'll let me out of her sight for a while."

" _You can always pretend amnesia."_ Ritsu pointed out. _"It's difficult to disprove, and if we let you reappear while you're still recovering, there will probably be symptoms to support the idea of head damage."_

"Then I'll be stuck in hospital for ages." Nagisa pointed out, grimacing at the mention of 'head damage'. "The bills will be horrible."

"Well, we'll have time to figure it out." The assassination instructor decided. "For now, we should brief the rest of the class quietly, and arrange a way for you to disappear."

"Are we telling Bitch-sensei?"

Karasuma grimaced. "If necessary."

" _Sensei, I can use the students' phones to tell them what's going on."_ Ritsu offered.

"That's probably the most secure way to do it. Thank you, Ritsu."

"…I usually walk home from the station, so if I leave late tonight from after-school lessons, it could be believable for me to disappear on the way home." Nagisa mused.

"Hm." Karasuma considered his words, his near-perpetual frown light and thoughtful. "I drive, so I'll get back first. I'll cover my license plates, disguise myself, and then drive to abduct you, and take a very circuitous route home. How does that sound?"

"…Terrifying, sensei." He replied, quite honestly. He wondered whether being able to make abduction plans on the fly was a special forces thing, or just a Karasuma-sensei thing. He had very little idea what the man had done for a living before this, after all.

"Then it's settled. I'll put the case into my car until then." Karasuma nodded decisively. "You should return to class, Nagisa-kun. Perhaps try to kill the octopus when you arrive."

" _I'll coordinate with him, sensei."_

The two of them who were not artificial intelligences blinked with surprise as the middle panel of the case, as well as the locking panels, closed on their own with a few whirrs and clicks. The older man shrugged, and bent to pick the case up, utterly ignoring the wheels. Nagisa took a moment to feel jealous of his obvious strength.

"I'll see you later for after-school lessons, Nagisa-kun. Ritsu, try to brief everyone discreetly." He nodded to them, and left the room.

Nagisa looked down at his phone, feeling quite overwhelmed. "…Alright, what now?"

" _Now, I prepare my fixed artillery body, and you prepare your knife._ " Ritsu said. _"I have enough experience shooting at beings like sensei that I should be able to score some hits if you distract him."_

Back on the reassuring ground of sensei-killing, Nagisa felt some tension leave his shoulders. "That sounds good. Can you give me any tips?"

She giggled. _"Nope!"_

"Naturally." He sighed at her, slipping her into his pocket. "Alright, I'll try to get him when I apologise for missing class. Please be ready to support me with your guns."

" _Of course!"_

* * *

Naturally, he did not succeed at killing Korosensei.

Ritsu did hit two tentacles, though, so he counted it as a minor victory.

* * *

 _End chapter._

Hullo, this is my first story for this fandom. I don't quite have a grasp on the characters yet but I'm giving it a decent shot.

There are currently 4 chapters of this story on the internet - 1 here, 3 on ao3, and 4 on tumblr. I will update tumblr first and ao3 second, and then here. Updates will be sporadic. If you fancy reading ahead, my ao3 is Spontaneite and my tumblr tenspontaneite.

If you fancy leaving comments that would be nice.


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